DO NOT USE Bondo in this application!


been there, done that..............Bondo is not compatible w/ redwood.

I repaired a number of south facing redwood window sills (~ six sills)
(southern California, ~15 to 20 inches of rain, no freezing) with Bondo
& then painted w/ a high quality primer & oil base paint.


The results lookes fantastic!!!!.......for about a year or two.
Now 15 years later, sills are just as messed up as be I restored them


I did remaing sills (~12) a few months later, same process except I
used Abatron Wood Epox instead of Bondo, the sills are still in great
shape, need some minor sanding & repainting


http://www.abatron.com/home002.htm

or

http://www.smithandcompany.org/


Both companies make quality products, I used both.
Their stuff is not cheap but if you value your time, use it.

Both sell mail order, Smith may have a local desrtibutor in your area &
you might save some on shipping.

cheers
Bob